Optical fibers provide a means to transfer communication and data. Optical fiber transfer has many advantages over the traditional electrical, coaxial or hard-wired transmission systems. Light from a laser or light emitting diode (LED) is modulated very rapidly to encode large amounts of information for transmission by the optical fiber. The output of the modulated LED source is sent through the optical fiber to a receiver (a photo detector) where it is processed (demodulated) to recover the communication or data.
The structure of a typical optical fiber can be described as a layered fiber of glass with a small diameter. The central portion of the fiber is the core and is made up of one type of glass. The core is surrounded by a different type of glass, which is called the cladding. Finally, the glass is coated with a protective jacket. The light-guiding capability of the fiber is dependent upon the properties of the core and cladding while the mechanical strength of the fiber is maintained by the protective jacket.
Light is transmitted through the optical fiber by means of internal reflectance. The cladding material is selected so as to have a lower index of refraction than that of the core material. Light rays that strike the interface between the core and the cladding at angles greater than a critical angle (which is determined for each combination of glass) are reflected back into the core, and through successive reflections are transmitted to the output end of the fiber. Light striking the interface at less than the critical angle is partly reflected and partly refracted into the cladding which results in a partial loss in signal intensity.
A significant consideration in fiber optics is the transmissivity of the fiber, i.e., its ability to propagate light of a given wavelength along the core with a minimum loss of intensity from the input end to the output end of the fiber. The light loss is described as the attenuation rate, expressed in dB (decibel) per kilometer of fiber. Glass fibers are used extensively, particularly for long distance transmission, such as in long-distance telephone lines, which require high transmissivities, and in sensor applications.
A single glass fiber is capable of replacing a very large bundle of individual copper wire. For example, a typical telephone cable may contain over 1,000 pairs of copper wire and have a cross-section diameter of three to four inches. A single glass fiber core/cladding cable capable of handling the same amount of signal might be only one-fiftieth inch in diameter. The core and cladding diameters can vary depending on a particular application. A glass fiber is typically 125 microns in diameter.
Glass optical fibers also find use in illumination applications for transmitting visible light to remote sites where it may be difficult to locate and service a more conventional light source, e.g., light bundles in endoscopes or in environments where electrical sparks could be hazardous.
Glass optical fibers are typically constructed from doped silicas and are very fragile. In addition, their use temperatures are limited to the thermal stability of the protective jacket (coating). Many uses for optical fibers demand higher operating temperatures, for example, sensor applications for oil well exploration and other geothermal applications, sensors for internal combustion engines, and in uses where fire/thermal resistance is important. Typically, polymers having silicone, isocyanate, or acrylate functionalities are used as protective jackets (coatings) on glass fibers. These polymers limit the use of glass fibers to temperatures between 100.degree. C. and 150.degree. C.
Polyimide coatings which are applied as a polyamic acid provide a less satisfactory protective jacket than a polyimide coating of the present invention. For example, a polyamic acid solution is applied at a lower solids content than a polyimide. Therefore, each application of the polyamic acid is a thinner coat than would be applied as a polyimide. A polymer solution having a solution (Brookfield) viscosity of about 8,000 to 10,000 centipoise will generally give satisfactory coating behavior on glass fiber (as compared to acrylate technology). However, the solids content of the polymer solution directly affects the solution viscosity of the polymer. Many conventional polyamic acids have unstable solution viscosities and are typically applied in several layers to achieve a particular thickness. In these multipass manufacturing operations, the desired coating thickness (generally 5 microns or more) is built up through several passes through the manufacturing process.
Solvent removal is another area of concern. The high boiling aprotic solvents used for polyamic acid preparation require high temperatures to remove solvent residue from the polyamic acid. In addition, polyamic acids are generally imidized at high temperatures to form the polyimide. During these high temperature (thermal) imidization processes water of cyclization is released, increasing the potential for bubbles or blisters to form in the polymer coating which can affect the mechanical characteristics of the coating and may interfere with light transmission through the glass fiber. In addition, the solvent removal and imidization steps attributable to the polyamic acid contribute to a slow and expensive manufacturing process because they require long residence times in ovens at high temperatures. On the other hand, fiber coating must be a rapid manufacturing process so that the polymer coating can be applied evenly and efficiently.
A rapid continuous manufacturing process can be achieved by using the polyimide of the present invention where the coating can be applied to the glass fiber, the solvent can be removed and the polyimide can be crosslinked by exposure to actinic radiation at low temperatures in a single pass through the manufacturing process.
Further, a useful polymer coating for glass fiber must have high temperature resistance and have good adhesion characteristics, but still maintain good stripping characteristics. In addition, the coating material must have a low coefficient of linear thermal expansion (CLTE) so as to more closely match that of glass. Otherwise, two different types of coating may be required, one coating to function as a low modulus buffer layer, and the other coating to function as a harder physical barrier against applied stresses from the environment.
The polyimide coated glass fibers of the present invention provide the advantageous characteristics described above. In particular, the coated glass fiber of the present invention provides a glass fiber that has a mechanically strong jacket and that can be exposed to high temperature applications.
Further, the application of the polyimide of the present invention to a glass fiber provides manufacturing advantages. The ability to apply the polyimide to the glass fiber at high solids content means that a thicker coat can be applied and the desired thickness can be achieved with fewer passes, ideally with one pass. Prior art coatings are able to apply approximately 1.5 microns per pass, whereas the present invention can provide up to 10 microns per pass.
Still further, the polymer is applied as a polyimide and although the polyimide coating is exposed to actinic radiation in order to crosslink the polyimide, no further imidization is required. Therefore, prior art problems associated with water formation during the imidization process are avoided. In addition, the polyimide of the present invention is soluble in low boiling point solvents, for example, dichloromethane. Therefore, residual solvent removal is rapid and can be accomplished with a low temperature oven or under the low temperatures associated with UV exposure.